1. Field of the Invention
This invention relates to a method of transferring Bloch lines present in the magnetic wall of a magnetic section formed in a magnetic thin film along the magnetic wall.
2. Related Background Art
Today, various memory devices, such as magnetic tapes, Winchester discs, floppy discs, optical discs, magneto-optical discs and magnetic bubble memories, are used as memories such as outside memories for computers, memories for an electronic file and memories for a still picture file. Among these memory devices, memories other than magnetic bubble memories require the recording-reproducing head to be moved relative to the memory during the recording and reproduction of information. That is, with such relative movement of the head, rows of information are fixedly recorded on information tracks by the head or rows of information fixedly recorded on the information tracks are reproduce by the head.
However, as a higher degree of recording density has been gradually required in recent years, there have arisen various problems. That is, tracking control for causing the head to accurately follow the information tracks has become complicated, the quality of recorded and reproduced signals has deteriorated, because of the control being insufficient, the quality of recorded and reproduced signals has deteriorated by vibration of the head moving mechanism or by dust or the like adhering to the surface of the memory, and further, in the case of a memory in which recording and reproduction are effected with the head being in contact with a magnetic tape or the like, abrasion has been caused by sliding. In the case of a memory, such as an optical disc in which recording and reproduction are effected with the head being in non-contact with the disc, highly accurate focusing control for focusing has become necessary, and the quality of recorded and reproduced signals has deteriorated because of the control being insufficient.
On the other hand, the magnetic bubble memory, as shown in the applicant's U.S. application Ser. No. 801,401 filed Nov. 25, 1985 now abandoned, can effect recording of information at a predetermined position and transfer the recorded information and can reproduce the information at a predetermined position while transferring the information and therefore, does not require relative movement of the head during recording and reproduction. Therefore, it does not pose the problems noted above, even when increasing the recording density in degree, and it can realize high reliability.
However, the magnetic bubble memory uses, as information bits, circular magnetic sections (bubbles) created by applying a magnetic field to a magnetic thin film, such as a magnetic garnet film having a readily magnetizable axis in a direction perpendicular to the film surface thereof and therefore, even if use is made of a minimum bubble (diameter 0.3 .mu.m) limited from the material characteristic of the present-day garnet film, several tens of M bits/cm.sup.2 is the limit of the recording density and a higher degree of recording density is difficult to realize.
So, recently, attention has been paid to a Bloch line memory as a memory having a recording density exceeding the limit of the recording density of the above-described magnetic bubble memory. This Bloch line memory, as shown in U.S. Pat. No. 4,583,200, uses, as information, bits of a pair of Neel magnetic wall structures (Bloch lines) interposed between Bloch magnetic wall structures existing around a magnetic section created in a magnetic thin film and therefore, it permits the recording density to be increased by approximately two factors as compared with the above-described magnetic bubble memory. For example, when use is made of a garnet film of a bubble diameter of 0.5 .mu.m, a recording density of 1.6 G bits/cm.sup.2 can be achieved.
FIG. 1 of the accompanying drawings shows a schematic perspective view of an example of the magnetic material structure constituting a Bloch line memory.
In FIG. 1, reference numeral 2 designates a substrate formed of non-magnetic garnet such as GGG or NdGG, and a magnetic garnet thin film 4 is provided on the substrate 2. The film 4 can be formed, for example, by a liquid phase epitaxial growing method (LPE method), and the thickness thereof is, for example, of the order of 5 .mu.m. Reference numeral 6 denotes a stripe-like magnetic section formed in the magnetic garnet thin film 4, and a magnetic wall 8 is formed as a boundary with the other area of the magnetic section 6. The width of the stripe-like magnetic section 6 is, for example, of the order of 5 .mu.m, and the length thereof is, for example, of the order of 100 .mu.m. The thickness of the magnetic wall 8 is, for example, of the order of 0.5 .mu.m. As indicated by arrows, the direction of magnetization in the magnetic section 6 is upward while, on the other hand, the direction of magnetization outside the magnetic section 6 is downward.
The direction of magnetization in the magnetic wall 8 rotates as if it were gradually twisted from the inner surface (i.e., the surface adjacent to the magnetic section 6) side toward the outer surface side. The direction of this rotation is converse on opposite sides of the magnetic wall 8 with Bloch lines 10 present vertically in the magnetic wall 8 as the boundary. In FIG. 1, the direction of magnetization in the central portion in the thickness direction of the magnetic wall 8 is indicated by arrows, and the direction of magnetization in the Bloch lines 10 is likewise indicated.
A downward bias magnetic field H.sub.B is applied from the outside to the magnetic material structure, as described above.
As shown, there are two kinds of Bloch lines 10 differing in the direction of magnetization, and the presence and absence of a pair of Bloch lines are made to correspond to information "1" and information "0", respectively. A pair of Bloch lines exist in potential wells formed at a certain period along the magnetic wall 8. Also, a pair of Bloch lines are successively transferred to the adjacent potential well by applying a pulse magnetic field perpendicular to the surface of the substrate. Thus, recording of information into the Bloch line memory (writing of the pair of Bloch lines into the magnetic wall 8) and reproduction of the information recorded in the Bloch line memory (reading-out of the pair of Bloch lines in the magnetic wall 8) can be accomplished at respective predetermined positions, while the pair of Bloch lines are transferred in the magnetic wall 8. The recording and reproduction of information can be accomplished by applying to the end portion of the stripe-like magnetic section 6, a pulse magnetic field of predetermined intensity perpendicular to the surface of the substrate, and although not shown, as pulse magnetic field applying means for recording and reproduction, conductor patterns for the supply of pulses are formed on the surface of the magnetic thin film 4 in predetermined , positional relationship with the stripe-like magnetic section 6.
In the Bloch line memory as described above, the formation of the potential well for the pair of Bloch lines is accomplished by providing periodic regular patterns on the surface of the magnetic thin film so as to cross the magnetic wall.
FIG. 2 of the accompanying drawings is a fragmentary plan view of the Bloch line memory showing an example of such patterns.
In FIG. 2, a number of line-like patterns 9 extending across the stripe-like magnetic section 6 and parallel to one another are provided on the surface of the magnetic thin film 4. These patterns each comprise a layer of a conductor such as Cr, Al, Au or Ti, the width thereof is, for example, of the order of 0.5 .mu.m and the arrangement pitch thereof is, for example, of the order of 1 .mu.m. By magnetic strain based on the formation of such pattern-like conductor layers, potential wells can be formed in the magnetic wall 8 and moreover, the arrangement thereof can be made regular and periodic. As the patterns 9, besides the above-mentioned conductor layers, use may be made of magnetic material layers or ions such as H ions, He ions or Ne ions driven into the vicinity of the surface of the magnetic thin film 4 in the form of the patterns. Also, potential wells formed by these patterns are symmetrical with respect to the direction of transfer of the Bloch lines indicated by arrows.
Now, transfer of the Bloch lines is accomplished by applying a pulse magnetic field perpendicular to the film surface of the magnetic thin film 4, and moving the Bloch lines to the adjacent potential well by utilization of precessional movement of magnetization caused thereby. At this time, an asymmetrical pulse magnetic field as shown in FIG. 3 of the accompanying drawings is used as the perpendicular pulse magnetic field H.sub.P to thereby irreversibly transfer the Bloch lines in a particular direction.
As described above, in the conventional Bloch line transfer, utilization is made of the transient response characteristic of the magnetization by the application of a perpendicular pulse magnetic field and therefore, there arises a problem that if the wave form of the pulse magnetic field is slightly disturbed or the magnetic thin film 4 has a defect, accurate transfer of the Bloch lines becomes difficult.